1. Field of the Invention
The invention relates to a process for preparing alkylchlorosilanes from the liquid constituents of the residues from the direct synthesis of alkylchlorosilanes.
2. Background Art
In the direct synthesis of alkylchlorosilanes of the general formula RaHbSiCl4xe2x88x92axe2x88x92b where a is 1-4 and b is 0, 1 or 2 from silicon metal and alkyl chlorides Rxe2x80x94Cl, where R is methyl is particularly preferred, oligosilanes, carbosilanes, siloxanes and high-boiling decomposition products are formed as by-products. Furthermore, very fine solids from the direct synthesis which are not retained by cyclones and filters are also present in the distillation residue. The solids comprise silicon, metal chlorides, e.g. AlCl3, metal silicides and carbon.
The predominant part of these residues constitute oligosilanes, in particular the disilanes RcCl6xe2x88x92cSi2, where c is 0-6. For this reason, processes which convert the disilanes into monosilanes have been developed. Conversion may be achieved, for example, by amine-catalyzed cleavage with hydrogen chloride. However, only disilanes having fewer than 4 methyl groups can be cleaved by this process. Furthermore, the disilanes must be separated beforehand from the solid residues, since these residues, for example aluminum chloride, act as catalyst poisons.
In order to also recycle uncleavable disilanes, processes in which uncleavable disilanes are converted into cleavable disilanes and then cleaved, e.g. as described in U.S. Pat. No. 4,393,229, or in which these disilanes are cleaved directly by means of HCl over specific catalysts, usually comprising noble metals, e.g. as described in U.S. Pat. No. 5,502,230, have been developed. A disadvantage of metal-catalyzed reactions is always the tendency for the catalysts to be poisoned by impurities from the residue.
U.S. Pat. No. 2,681,355 describes a process in which relatively high-boiling residues from the direct synthesis of methylchlorosilanes are converted into monomeric silanes purely thermally, in the absence of catalysts, by employing hydrogen chloride at temperatures of 400-900xc2x0 C. in an autoclave or in a tubular reactor. The patent emphasizes the low tendency to form carbon deposits as a great advantage when using a tubular reactor, as a result of which the process is said to be operable for a prolonged period. However, this does not apply when treating residues which, because of their solids content, quickly lead to blockage of reaction tubes.
U.S. Pat. No. 5,877,337 describes a process which allows even solids-containing residues from the direct synthesis of organochlorosilanes to be worked up at low pressures and yet allows the organosilicon components to be converted into useful silanes. This objective is achieved by thermal cleavage of the direct synthesis residues with hydrogen chloride at temperatures of 300-800xc2x0 C. in a tubular reactor having rotating internals. The rotary motion of the internals strips off caked deposits on the reactor walls caused by carbonization or by deposition of solid components, thus preventing blockage of the reactor. However, this process is technically difficult to implement and is therefore correspondingly costly.
U.S. Pat. No. 5,592,912 describes a process for the cleavage of direct synthesis high-boiling residue with HCl in the presence of catalysts such as activated carbon, or supported aluminum chloride, platinum, or palladium compounds. Supports described are aluminum oxide, silica, zeolites and active carbon. The process can be carried out as a fixed-bed process or in a fluidized bed.
The preparation of trichlorosilane and tetrachlorosilane mixtures from silicon and hydrogen chloride in a fluidized bed has been described, for example, in U.S. Pat. No. 4,130,632.
The processes mentioned above suffer from the disadvantage of a high outlay in terms of apparatus required to solve the carbonization problem or the use of specific catalysts, sometimes based on noble metals, which make the process more expensive.
It is an object of the invention to provide an improved process which allows work up of solids-containing residues from the direct synthesis of alkylchlorosilanes, and which allows the organosilicon components to be converted into usable silanes.
The invention provides a process for preparing alkylchlorosilanes from the liquid constituents of the residues from the direct synthesis of alkylchlorosilanes which have a boiling point above 70xc2x0 C. at 1013 hPa and comprise disilanes, in which the residues are heated with hydrogen chloride and silicon at temperatures of at least 300xc2x0 C., wherein at least 10% by weight of trichlorosilane and/or tetrachlorosilane, based on the weight of the alkylchlorosilanes formed, are formed concurrently. The process is, in particular, simple to carry out, and allows very long running times.
Hydrogen chloride and Si metal react in an exothermic reaction to produce a mixture of trichlorosilane and tetrachlorosilane. The equilibrium composition of the mixture is dependent on the reaction temperature. The reaction enthalpy is utilized to heat the liquid constituents of the residues from the direct synthesis of alkylchlorosilanes fed into the reactor to reaction temperatures at which the cleavage by means of hydrogen chloride to form monosilanes then takes place. In the process, it is preferred that at least 20% by weight, in particular at least 30% by weight, of trichlorosilane and/or tetrachlorosilane, based on the weight of the alkylchlorosilanes formed, are formed at the same time. The product obtained is a mixture of trichlorosilane and tetrachlorosilane together with varying proportions of organochlorosilanes, depending on the composition of the liquid constituents of the residues. The silicon is preferably present as a fixed bed or, in particular, in a fluidized bed in the process.
The amount of HCl can be varied within a wide range and, in the case of fluidized-bed reactors, the lower limit is determined essentially by the minimum amount required for maintaining the fluidized bed, while the upper limit is determined essentially by the need to avoid excessive discharge of dust from the fluidized bed. Both are highly dependent on the particle size and the geometry of the reactor. In fixed-bed reactors, the amount of HCl is limited by plant engineering considerations and economic constraints, e.g. by the loss of HCl. The amount of hydrogen chloride used is at least the molar equivalent amount of the disilanes present in the residue and the trichlorosilanes and/or tetrachlorosilanes to be formed. The residue and the hydrogen chloride may both be preheated, or may be metered at ambient temperature into the reactor, with the streams preferably being metered in continuously. When heating is utilized, the residue can also be metered in as a gas/liquid mixture. Preference is given to using a fluidized-bed reactor. Preference is further given to using customary plants for the preparation of trichlorosilane and tetrachlorosilane.
The alkylchlorosilanes prepared by cleavage of disilanes from the liquid constituents of the residues from the direct synthesis of alkylchlorosilanes preferably have the above general formula where R is a methyl, ethyl, butyl or propyl radical, in particular a methyl radical. Most preferably, methyltrichlorosilane, dimethylchlorosilane, trimethylchlorosilane, methyldichlorosilane, or dimethylchlorosilane are prepared.
The target product of the direct synthesis of methylchlorosilanes is generally dimethyldichlorosilane which has a boiling point of 70xc2x0 C. and usually makes up more than 80% of the crude mixture and is purified by distillation. The residues from the direct synthesis having a boiling point above 70xc2x0 C., preferably at least 80xc2x0 C., in particular at least 100xc2x0 C., comprise, inter alia, monosilanes, e.g. ethyldichlorosilane, ethylmethyldichlorosilane, ethyltrichlorosilane or i-propyltrichlorosilane. The proportions can fluctuate greatly and are usually less than 10% of the residue. Greatly fluctuating proportions of disilanes, e.g. hexamethyldisilane, pentamethylchlorodisilane, 1, 1 -dichloro-1,2,2,2-tetramethyldisilane, 1 ,2-dichloro- 1,1,2,2-tetramethyldisilane, 1,1,2-trichloro-1,2,2-trimethyldisilane, 1,1,1-trichloro-2,2,2-trimethyldisilane, 1,1,1,2-tetrachloro-2,2-dimethyldisilane, 1,1 ,2,2-tetrachloro-1,2-dimethyldisilane, pentachloromethyldisilane and hexachlorodisilane, make up the major part of the residue. The proportion of disilanes is usually 20-80%, most often 40-80% of the residue. Further constituents are carbosilanes, oligosilanes having three or more silicon atoms, hydrocarbons, and chlorinated hydrocarbons whose identification and quantification present great difficulties.
In addition, the proportion of solids can be up to 10% by weight. These include finely divided or very finely divided solids, for example, silicon and suicides, generally contaminated with catalyst and promoter residues, metal chloride mixtures comprising main constituents such as AlCl3, CaCl2, CuCl, and also including carbon. A detailed description of the residues is given, for example, in U.S. Pat. No. 5,292,912. The process is preferably carried out in the presence of the solids, since these do not adversely affect the process.
The silicon can be used in milled form or in lump or granular form. It is preferably used in excess for the preparation of trichlorosilane and/or tetrachlorosilane. It is preferable for further silicon to be fed in continuously.
The amount of liquid constituents of the residues from the direct synthesis of alkylchlorosilanes which is fed to the reaction depends on the need to maintain both the necessary temperature in the reactor, or the fluidized bed, if the latter is used.
The mixture obtained in the reaction and leaving the reactor is condensed, if appropriate freed of solids, and may be returned in whole or in part to the silane mixture produced in the direct synthesis, or may be fractionated separately into pure substances.
The components of the residue which are not cleavable under the conditions of the process, e.g. metal salts and other solids, generally deposit on the silicon surfaces and are finally discharged together with very finely divided silicon in the reaction gas, from which they are separated by means of devices such as filters or cyclones. However, the uncleavable components may also remain in the reactor, from which they may then periodically be discharged, together with the residual contents of the reactor. The solids in both cases can, after suitable treatment, be used as additives in the ceramics industry or as slag formers. If the metal content is sufficiently high, it may also be worthwhile to isolate these metals from the solids; for example, copper can be leached out and returned to the materials circuit.
The process is preferably carried out at temperatures of at least 400xc2x0 C., preferably at least 450xc2x0 C., and not more than 1200xc2x0 C. At temperatures higher than 600xc2x0 C. in the reactor, elimination of silicon-bound alkyl groups begins to occur in competition to the cleavage, resulting in formation of chlorodisilanes, and chlorosilanes such as trichlorosilane or tetrachlorosilane. This reaction can be exploited to keep the cost of distillation for fractionating the product mixture low. However, preference is given to obtaining the alkylchlorosilanes in admixture with trichlorosilane and tetrachlorosilane, and using the alkylchlorosilanes obtained after purification by distillation for preparing silicone products such as polydimethylsiloxane or silicone resins.
The pressures prevailing in the reactor during the process can be varied within a wide range and are preferably in the range of 1-5 bar (abs.). Particular preference is given to operating the reactor fully continuously, with both the residues and the silicon being added continuously. If desired, a plurality of introduction points for a given stream, e.g. residues or HCl, can be provided along the length of the reactor.